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natural products are all active. One of the great achievements
of Pasteur in this line of research was the separation of inactive
"externally compensated" compounds into their active com-
ponents or "optical antipodes" or " enantiomorphs." One
method of separation is described in Prep. 35. For details of
other methods a book on stereo-chemistry must be consulted.

On the formation of ethyl tartrate, see notes on Prep. 15, p 247.
Ethyl tartrate may also be obtained by the method described
in Prep. 86, which rather curtails the operation and does not
necessitate the use of more than half the quantity of ethyl
alcohol required by the earlier process.


Racemic and Mesotartaric Acids. These two acids
represent two inactive types of compounds containing' asymmetric
carbon atoms (see above). Apart from certain well-marked
differences in physical properties they also differ in one
important feature ; racemic acid can be resolved into its optical
enantiomorphs, whereas mesotartaric acid cannot. The latter
belongs to what is termed the inactive indivisible type. If we
examine the structural formula of tartaric acid it will be seen
that it possesses two asymmetric carbon atoms, denoted in the
formula by thick type.



J                           Each asymmetric carbon atom is attached to similar groups.
r| '                      Let us suppose that each asymmetric carbon with its associated
fl I                   groups produces a certain rotation in a given direction.    We
*j j                        may imagine the following combinations of two similarasyrnmetrie
||! {                       groups.    Both produce dextro-rotation, or both produce laevo-ro-
| j                        tation. They will represent the dextro andlaevoenantiomorphs,ancl
fyj                       the mixture of the two will produce inactive racemic acid. Racemic
I li                       acid is said to be inactive by external compensation.    Suppose,
|l|                       finally,  that the two asymmetric groups produce  rotation   in
I j                        opposite directions.     They will neutralise one another.    The